Question

Please help! Not even sure how to start with this one or how to calculate

Answer 1

Remember when you learning about the "Least Common
Multiple" and you wondered when in the world you would
ever need to use it ?
Well, here you are !   Hello !

The next time they will cross the starting line together is the
LCM (Least Common Multiple) of 30 and 40 .

Do you remember how to find it ?
Here's one easy way: 

Count by 30s until you come to a number that's divisible by 40 .

Laura's first lap. . . . . . . . 30 seconds . . . not divisible by 40
Laura's two laps . . . . . . . 60 seconds . . . not divisible by 40
Laura's three laps. . . . . . 90 seconds . . . not divisible by 40
Laura's four laps. . . . . . 120 seconds . . . divisible by 40

After 120 seconds, Laura has completed four laps,
Shelby has completed only three laps.